Related subject matter is disclosed the concurrently filed application entitled xe2x80x9cPOLARIZATION DIVERSITY FOR BIREFRINGENT FILTERSxe2x80x9d by J. N. Damask and C. R. Doerr, which is assigned to the same Assignee.
The invention relates generally to birefringent filters and, more particularly, to a double-pass polarization diversified birefringent filter for separating even and odd channels of a wavelength division multiplexed signal.
The birefringent filter was first developed by astronomers at the turn of the century so that they could make celestial observations of the Sun through a narrow-band spectral window. The classic papers on these filters were written by Loyt [1] Evans [2] and Solc [3]. (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) In 1965 Harris et al [4] published a method of filter synthesis for the birefringent filter. In the late 1980s, Buhrer [5] brought the birefringent filter into the optical communications world with his demonstration of such a filter designed for optical wavelength multiplexing and demultiplexing.
The filters build by Lyot, Evans, and Solc, FIG. 1a used polarizers placed before and after the birefringent filter proper to isolate one polarization state at the input and then discriminate the state of polarization at the output of the filter. This was sufficient for their application because the Sun is an intense source and the polarization of light from the Sun is completely random.
In the late 1980s, Buhrer recognized that the use of input and output polarizers created excessive polarization-dependent loss and was therefore unsuitable for telecommunication applications. He instead created a method for polarization diversity. New input and output stages were proposed to allow the birefringent filter to handle simultaneously two orthogonal polarizations and to allow the output of the filter to couple to two complimentary ports. In his scheme, there is no loss of light, regardless of polarization. Buhrer U.S. Pat. No. 4,987,567 [6] further describes the use of a birefringent filter as an optical signal multiplexer/demultiplexer.
This invention makes two principal material improvements to the Buhrer scheme. First, improved input and output polarization diversity apparatuses to replace those invented by Buhrer are disclosed. Second, the architecture of the filter disclosed herein is folded in a compact manner to produce a simple double-pass filter. This is both separate and inventive on the Buhrer patent.
The double-pass birefringent filter disclosed herein offers a decreased polarization sensitivity and an increased crosstalk rejection over prior art designs and offers low insertion loss, high contrast, and low chromatic dispersion. Moreover, the double-pass design, in contrast to a single-pass design, offers improved filtering over prior art designs. A central application of the double-pass birefringent filter is for use as an interleaving filter to separate even and odd channels of a wavelength-division multiplexed optical signal.
In accordance with the present invention, a double-pass birefringent filter apparatus comprises an external-interface apparatus, itself illustratively comprised of a beam-displacer element and a waveplate element; a birefringent filter comprised of a series of several birefringent waveplates; and a wrap-around unit, itself illustratively comprised of a beam-displacer element and a retro-reflector. In response to an input beam received at the external-interface apparatus, the double-pass filter forms two output beams which exit the external-interface apparatus in a direction opposite that of the input. The output beam intensities vary in a complimentary manner so that their sum power remains invariant to the input frequency.
More particularly, our double-pass periodic birefringent filter comprises (1) an external-interface apparatus for receiving an input optical beam and producing therefrom parallel first and second polarized optical beams travelling in the forward direction offset in a first direction and having matching linear polarization states; (2) a birefringent filter apparatus for receiving the first and second polarized forward-going optical beams and producing therefrom first and second elliptically polarized optical beams, each including first and second orthogonal beam components, the beams having complimentary intensities which vary periodically, over a predefined free-spectral range, with the frequency of the input optical beam; (3) a wrap-around apparatus for further separating in a second direction first and second elliptically polarized beams into four beams all with linear polarization states, and for reversing the direction (hereafter referred to as the backward direction) of all four linearly polarized beams so as to transit again the birefringent filter apparatus; where the clear aperture of the filter plates of the birefringent filter apparatus is large enough to accommodate both the forward-going beams and the backward-going beams, (4) where the same birefringent filter apparatus produces from the backward-going beams four elliptically polarized optical beams, each including first and second orthogonal beam components; and (5) where the external-interface apparatus has large enough clear aperture so as to accommodate all four backward-going beams (in addition to the two forward-going beams), and produce therefrom six parallel optical beams, two of which possess, in general, the two complimentary orthogonal polarization components (these beams being collected), and four of which possess, in general, only one of the two complimentary orthogonal polarization components (these beams being rejected).
In the preferred embodiment, the external-interface apparatus includes a birefringent beam displacer and a xcex/2 waveplate which is offset so as to intersect only one half of the forward- and backward-going beams; the birefringent filter apparatus includes one or more birefringent waveplates designed so that the thickness of the plates controls the free-spectral range of the filter and the relative azimuth orientation of the plate(s) controls the shape of the filter intensity response; the wrap-around apparatus includes a birefringent beam displacer rotated 90 degrees with respect to the birefringent beam displacer located in the external-interface apparatus about the axis of the forward-going optical beam(s), and a retro-reflector mirror. In other embodiments a rhombus polarizing beam splitter (PBS) may be substituted for the birefringent beam displacer and/or a right-angle prism may be substituted for the retro-reflector mirror.